Many types of photographic films, for example, x-ray films, graphic arts films, and data recording films, employ a web comprising one or more polyester polymers as the film support. Typical polyesters are polyethylene terephthalate ("PET") and polyethylene naphthalate ("PEN"). Various copolymers and blends of polyesters, or of a polyester polymer with one or more non-polyester polymers, are also known in the art.
In manufacturing biaxially-oriented film base, thermoplastic resin typically is extruded as a relatively thick, high-viscosity, molten ribbon onto a moving receiver surface, typically a polished casting wheel. The temperature of the ribbon may be adjusted, and then the ribbon is stretched in the machine direction (MD orientation), or "drafted", and stretched in the transverse direction (TD orientation), or "tentered" in known fashion to biaxially orient the molecules of the polymer and to achieve the desired final width and thickness of the ribbon as a web or sheet. The longitudinal stretching step can be performed before or after the transverse stretch step. As used hereinbelow, "sheet" can mean either continuous planar polymeric material, commonly referred to as "web," or discrete sections thereof. To enhance the crystallinity and to increase the dimensional stability of the web, the biaxially-oriented polymeric web is "heat-set" by heating it above its glass transition temperature (T.sub.g) to near its crystallization point, while maintaining the web under constant tension. The heating and tensioning also ensure that the heat-set film remains transparent upon cooling. To reduce residual stresses and improve planarity after being heat-set, the web may be subjected to a period, typically several minutes, of temperature above T.sub.g but below the heat-set temperature, a process known as "heat relaxation". Typically, the web is rapidly cooled following each of the heat set and heat relax steps to lock in the desired properties. Following heat relaxation, the web is wound into a stock roll of desired length in preparation for subsequent coating of photographic layers. Details of the manufacture of polyester webs are disclosed in, for example, U.S. Pat. Nos. 2,779,684, and 4,141,735.
For some applications, the wound roll of web must also be "annealed" by incubation at elevated temperature for a period, typically, of days to minimize an inherent tendency to adopt an undesirable level of curl when wound in a cassette or camera (so-called "core-set curl"), as disclosed in U.S. Pat. No. 4,141,735. Annealing is particularly effective in preparing polyethylene terephthalate and polyethylene naphthalate for use as a photographic support.
A serious problem in the use of polyester webs for photographic film support is the generally repellent nature of the web surface to the coating and adherence of aqueous subbing layers or emulsions. This problem is overcome typically by the coating of a special chlorine-containing latex undercoat ("U-coat" or "primer") on the polyester surface prior to the coating of a subbing layer or photographic emulsions. Typically, adhesion-promoting layers are applied at an in-line coating station in the base making machine before cutting of the web and winding of the original stock rolls.
A limitation of this known technology is the requirement of coating the U-coat layer. This step introduces opportunities for coating defects and requires a corresponding drying section in the manufacturing line. For PEN film base manufacturing there is the additional limitation that the U-coat must be stable to annealing conditions.
Thus a need exists for an improved method for manufacturing polyester film base, wherein the web does not need to be coated with a U-coat. This is particularly needed for PEN film base that is subjected to annealing conditions.
It is a principal object of the invention to provide an improved method for making film base comprising polyester polymer, and particularly polyethylene naphthalate film base having acceptable planarity, core-set curl, and photographic layer adhesion.
It is a further object of the invention to provide an improved method for making polyester film base, and particularly polyethylene naphthalate film base, having acceptable photographic layer adhesion without resort to an undercoat.